The popular Democratic said in an emotional impromptu news conference outside his home before heading to City Hall that it was "a very difficult decision."

A formal announcement is scheduled for 4 p.m. EDT Thursday.

Menino has been mayor since 1993 and said he's lived the job "24-7."

"It's a changed city, and I'm glad to be a small part of this changed city," he said, crediting his staff and cabinet. He said the city is more tolerant, vital and with a younger population than when he took office.

The 70-year-old mayor was hospitalized for eight weeks in the fall after a respiratory infection and a blood clot that was complicated by a spinal fracture and diabetes. He said his health played a minor role in his decision.

He told WBZ-AM that he plans to go "full tilt" until his last day in office.

After that, his plans are unclear. He said he's looking forward to spending more time with his grandchildren, but insisted he is not retiring. He told the station he'd like to stay involved with the city's public schools.

His decision not to run again is expected to trigger a political scramble to replace him as a new generation of political figures eye the mayor's office.

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City Councilor John Connolly announced his mayoral intentions last month, regardless of Menino's decision. But Menino had been considered a heavy favorite had he opted to run.

As recently as January, Menino delivered an upbeat assessment of the city during his annual state of the city address.

Menino used a cane to walk to the podium and spoke vigorously about his plans for Boston. At the time, Menino gave no indication of whether he'd decided to seek a sixth term this year.

"Our progress is real. Our future is bright. The state of our city is striking, sound and strong," he said in prepared remarks that cited progress on economic development and crime reduction.

On Tuesday, Menino appeared at a rally at Boston City Hall plaza to urge the U.S. Supreme Court to repeal the federal Defense of Marriage Act. The previous day, he delivered a speech to the Boston Municipal Research Bureau in which he said he had never been more confident about the city and announced several new development projects and initiatives.

Menino became acting mayor after his predecessor, Raymond Flynn, left office in 1993 after being named ambassador to the Vatican. Menino, then president of the City Council, was automatically elevated to the mayor's job.

The circumstances prompted some critics to label him the "accidental mayor," a charge the sometimes-thin-skinned Menino was quick to reject. But he was elected mayor in his own right in November 1993 and won re-election by wide margins in 1997, 2001, 2005 and 2009.

The city's previous longest serving mayor, the late Kevin White, was in office for four terms, from 1968 to 1984.

Menino's longevity also exceeded the legendary Mayor James Michael Curley, who also served four terms, but not consecutively.

Menino built his reputation by focusing on the unglamorous nuts and bolts of running a major metropolitan city - fixing potholes, cleaning streets, even curbing the practice of saving a shoveled-out parking space by putting folding chairs or trash cans along the curb.

It's everyday commitments like those that earned him the nickname of the "Urban Mechanic."

The 2004 Democratic National Convention put Menino's political and negotiating skills to the test when the city's main police union threatened to picket over an unresolved contract.

It was only with the last minute help of other politicians, including Republican Gov. Mitt Romney, that a contract was reached in the early morning hours the day before the convention opened in the city.

During his years in office, Menino also became a vigorous national voice in favor of stricter gun control measures.

He co-founded Mayors Against Illegal Guns with New York Mayor Michael Bloomberg and criticized the National Rifle Association's call for more armed guards at schools after the Connecticut school shooting in December.

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